gregory david collins

Building a website with Haskell, part 2

Mar 30, 2009

In the second part of the series, we discuss the design of
this happstack
website.

In the last post in this series, I discussed why I chose happstack to power this website. In this post, I’ll describe its design. If you’d like to follow along, you can browse the source at my github page.

Types

Let’s begin by looking at the Homepage.Types module, which contains types and functions pertaining to the global website state; here “global” means “state that is shared by all requests”.

The datatype that holds the state for the site is called HomepageState:

It holds the contents of my delicious feed (which we’ll discuss later), and the HStringTemplate templates for the site. The delicious state is wrapped in an MVar; an MVar is a Haskell concurrency primitive that provides synchronized access to its underlying type. We use this to enforce mutual exclusion when accessing delicious, so that we don’t contact it more than once at a time.

The homepage state will, of course, be used by most of the URL handlers for the website. In order to provide easy access to this value so that we don’t have to pass it around everywhere, we’ll use a standard Haskell trick and wrap it in a reader monad:

typeHomepageMonad=ReaderTHomepageStateIO

Terminology like “reader monad” tends to frighten off newcomers but really all this is doing is allowing us to write:

foo ::HomepageMonadfoo =do homepageState <- ask...

We inject a HomepageState value into the monad when we evaluate it, and from then on we can chain monadic actions together without having to explicitly pass the state around. The “ReaderT” type is a “monad transformer”; that’s another $2 term that just means that it wraps an existing monad, producing a new monad that does everything the wrapped monad does, plus carrying some state around. In this case the wrapped monad is “IO.”

(A parenthetical: v0.1 used StateT here, which is clearly wrong.)

The next type definition covers the type of our URL handlers, or “server parts”:

typeHomepageHandler=ServerPartTHomepageMonadResponse

Happstack URL Handlers

Happstack URL handlers have the weird type ServerPartT m a — this is just a wrapper around the monad transformer type:

ReaderTRequest (WebT m) a

The ReaderT Request part here just means that you can get at the HTTP request with ask, and WebT is itself a monad transformer which takes a monad you give it and wraps its result to be of the form:

(Maybe (EitherResponse a, FilterFunResponse))

This is a little abstruse, and the happstack docs don’t do a great job of explaining it, which makes it tough to understand at first for n00bs like me with Master’s degrees in type theory. Some things you can do with a WebT:

short-circuit the computation with “mzero”, which is represented by a return value of Nothing. This is what allows you to chain handlers together. Happstack will try your handlers in order, and the first handler that returns a non-Nothing value will be used to satisfy the request. The mzero value is WebT’s way of saying “I choose not to handle this.”

short-circuit the computation with “finishWith response”, which is represented by a return value of Just (Left response, ...). This allows you to bomb out early with a response, ignoring any subsequent chained monadic actions.

add a filtering function to the handler with setFilter or composeFilter, which corresponds to a return type of Just (..., f), where f is (roughly) a function of type Response -> Response. Ignore the baffling SetAppend (Dual (Endo a)) type; that’s just there to dictate how filter functions get bolted together by their Monoid instance.

return a value of arbitrary type “a”, just like any other monad — that corresponds to a return type of Just (Right v, ...).

Happstack handlers belong to the Monoid typeclass. In case you didn’t do a math degree, a Monoid is a set with an associative binary operator, usually called “⊕”, and a zero element. In order for a set to be a monoid when it grows up, it needs to satisfy the “monoid laws”:

1. a ⊕ (b ⊕ c) = (a ⊕ b) ⊕ c

(associativity)

2. 0 ⊕ a = a

(left identity)

3. a ⊕ 0 = a

(right identity)

In Haskell, perplexingly, “⊕” is called “mappend” and “0” is called “mempty”. (This nomenclature comes from the monoid instance for lists.) The ServerPartT and WebT types have monoid definitions that allow you to chain handlers together. For instance, the expression:

(exactdir "/foo" fooHandler) `mappend` (exactdir "/bar" barHandler)

will cause fooHandler to be invoked if you request "/foo", and barHandler will be invoked if you request "/bar". (Note that exactdir comes from happstack-helpers.)

Site behaviour

Let’s take a look now at the Homepage.Handlers module, which defines the “toplevel” handler for the website:

It reads like it’s written: if the client requests the "/about" page, we serve the about.st template, using the posts.css stylesheet. I won’t go into HStringTemplate here, it’s a pretty typical templating engine; there’s some material on it in the happstack tutorial.

Something (slightly) non-trivial: integrating a delicious feed

I use delicious a lot, to record bookmarks I’m interested in across all of the computers I use. Since of course I only read turbo-interesting stuff that other people would be interested in, something I wanted to put on my front page was a syndication of the last few entries from this feed. Luckily Haskell already has a library to handle this; all we need to do is provide some plumbing. (It would be very easy to cook up something similar for twitter, assuming you can find any useful purpose for that dreck.)

First let’s take a look at the DeliciousState type:

dataDeliciousState=DeliciousState![D.Post] !UTCTime

To be nice to delicious, we’ll only pull my recent bookmarks once every four hours. DeliciousState is just the recent posts on the feed (the D.Post type comes from the Haskell delicious library) plus the timestamp of the last time we retrieved the feed.

I wanted to use relative time to present the bookmarks (e.g. “2 hours ago”). A DiffPost is just a delicious post plus a string containing the human-readable timestamp. The reason we can just splat the posts into the template directly is that we’ve told HStringTemplate how to encode it:

So getRecent just grabs the DeliciousState out of the HomepageState, passes it to getRecentPosts, and computes human-readable ages for the results. The getRecentPosts function is only a little bit hairier:

If the MVar is empty then haven’t yet pulled the feed and we should do so. If the MVar contains data, we check whether the last update was less than four hours ago, and if it was we just return the old posts. Otherwise, we fetch the feed and update the timestamp.

Next in this series

If you look at the source for this website, it’s clear that these posts are hard-coded as static content. In Part 3 of this series, we’ll create a simple dynamic content system which will allow us to drop posts on the filesystem, in markdown format, and have them be published to the site.